The present invention relates to a method of forming substrate electrodes of MOSIC (Metal-Oxide-Semiconductor Integrated Circuit), and particularly to a method of forming substrate electrodes through steps of a number as small as possible.
A silicon gate-type MOSIC having a thick oxide film which is selectively formed on a silicon semiconductor substrate except the regions where elements are to be formed, and having an interconnection formed on the thick oxide film, has been widely known as disclosed, for example, in "Philips Research Reports, Vol. 26, No. 3, pp. 157-165, June 1971". The so-called LOCOS (Local Oxidation of Silicon) type of MOSIC minimizes the stray capacitance in the wiring layer, enables the switching speed to be increased, and is advantageous for use with memory circuits.
With reference to the LOCOS-type silicon-gate MOSIC, a memory which employs polycrystalline silicon resistors as load resistances of transistors which constitute a memory cell, has been known as disclosed, for example, in U.S. Pat. No. 4,110,776 (Aug. 29, 1978). Since high reliability is required for the memory, the MOSIC must be packaged in the form of a ceramic package or a glass package instead of a resin mold package.
When a MOSIC device having electrodes on the back surface of the semiconductor substrate is to be sealed by packaging with ceramic material or glass, outgoing lead wires of the package and substrate electrodes must be connected by wires. With the electrodes being located on the back surface of the semiconductor substrate, however, it is very difficult to bond the wires between the substrate and the outgoing lead wires, and the technique of automatic wire bonding is not applicable.
To employ the ceramic packaging or glass packaging, therefore, it is necessary to provide a substrate electrode on the surface of the semiconductor substrate like other electrodes (bonding pads).
In particular, in dealing with the silicon gate MOSIC of the LOCOS-type, when the substrate electrode is to be installed on the surface of the semiconductor substrate like other electrodes, the source and drain are formed by introducing impurities of a conductivity type opposite to that of the substrate into the surface of the substrate except the areas of the semiconductor substrate covered with a pattern of thick silicon oxide (field oxide) and a pattern of silicon gate electrode. Therefore, after the impurities have been introduced, a portion of the pattern of the thick silicon oxide may be subjected to the etching to expose the substrate, thereby to form a substrate electrode on the exposed surface of the substrate.
To remove such a thick silicon oxide layer, however, a particular etching step is required which causes the yield to be decreased.